Sinosauropteryx caused a sensation when it was revealed to the world in 1996. Here was a clear and perfectly preserved skeleton of a small theropod dinosaur, but covering most of the body were impressions of an enigmatic fuzz, a coat of fine filaments up to 1 1/2 inches (4 cm) long.

Skin impressions found for some dinosaurs reveal that they had scales. But the growing recognition that birds evolved from dinosaurs raised the possibility that some dinosaurs had feathers, or at least some kind of protofeather-like body covering that later evolved into the feathers of present-day birds. However, feathers are relatively soft and rarely fossilized.

It is likely that several birdlike dinosaurs may have had feathers or similar structures, but, as they were not preserved, we know nothing about them. Then Sinosauropteryx was found in the fabulous Liaoning deposits in China. Here was the right kind of dinosaur (a small, advanced theropod) with much of its back, rump, and tail covered in a fine, filamentous fuzz that appeared to be fur-like coat made up of short, single strands.

While lacking the complex structure of a feather, it was a more intricate body covering than simple reptilian scales. The fuzzy coat seemed to form a downy layer that would have been perfect for trapping body heat and keeping the animal warm. This observation lends more weight to the ongoing debate that some dinosaurs were warm-blooded. It is also possible that the fuzz was used when displaying to attract mates.

In one of the specimens was the remains of its last meal, a small, unidentified animal-providing proof that this little ancestor of the birds ate the ancestors of modern mammals. One of the other specimens revealed unlaid eggs in the oviducts.